HOMOTHALL1SM AND HETEROTHALLISM 32<s 



assumption would account for the presence of two types of bi- 

 polar basidia in equal proportions, if reduction (segregation) 

 occurred in the first division and there was random segregation 

 without genetic linkage. With such genotypes a simple crossing- 

 over during meiosis would account for the tetrapolar basidia. 



Further light on this problem was shed by the studies of Sass 

 (1929). He found four-spored and two-spored forms of each of 

 the three species Coprinus ephemeras, Nancoria semiorbiciilatus, 

 and Galera tenera. The two-spored form of each is normally 

 homothallic. The four-spored forms are heterothallic and bi- 

 sexual, and sex is determined by one pair of Mendelian factors. 



Pamis stiptictis from Europe is non-luminous, but the same spe- 

 cies from North America is luminous. Studies by Macrae (1942) 

 of both European and North American strains of this fungus show 

 that each strain is heterothallic and tetrapolar. When she crossed 

 a luminous with a non-luminous one, the haploid mycelium of the 

 Fi generation was luminescent. Luminosity is therefore dominant 

 and is governed by a single pair of factors. 



hi Ustilaginales. Some of the more important contributions to 

 the genetics of smut fungi are those of Stakman and Christiansen 

 (1927), Christiansen (1929), Hanna (1929), Dickinson (1931), 

 Flor (1932), Allison (1937), Kernkamp (1939), and Schmitt 

 (1940). The smuts constitute a group of destructive plant para- 

 sites which, in regard to their sexual process, resemble the other 

 Basidiomycetes generally in that thev lack sexual organs and 

 definitive gametes. Their most distinctive feature, the mature 

 chlamydospore or smut spore, is uninculeate. Its nucleus is a 2n 

 structure. At germination meiosis occurs, and each haploid nu- 

 cleus finds its way into a basidiospore or sporidium. According 

 to Hanna (1929), infection of maize by Ustilago zeae is accom- 

 plished by haploid mycelia, and the chance meeting of two hap- 

 loid mycelia of opposite sex within the host tissues is followed by 

 hyphal fusions, whereupon the mycelial cells are dicaryotic. Pre- 

 viously Stakman and Christiansen (1927) had failed to obtain 

 fusions in artificial cultures between strains of opposite sex but 

 had found hyphal fusions and clamp connections in hyphae within 

 the maize tissues. Moreover, infections from monosporidial cul- 

 tures failed to result in the production of smut galls and of 

 chlamydospores. When dicaryotic mycelium eventually becomes 



